It has been customary in the electronic article surveillance industry to apply EAS markers to articles of merchandise. Detection equipment is positioned at store exits to detect attempts to remove active markers from the store premises, and to generate an alarm in such cases. When a customer presents an article for payment at a checkout counter, a checkout clerk deactivates the marker by using a deactivation device provided to deactivate the marker.
Known deactivation devices include one or more coils that are energizable to generate a magnetic field of sufficient amplitude to render the marker inactive. One well known type of marker (disclosed in U.S. Pat. No. 4,510,489) is known as a "magnetomechanical" marker. Magnetomechanical markers include an active element and a bias element. When the bias element is magnetized, the resulting bias magnetic field applied to the active element causes the active element to be mechanically resonant at a predetermined frequency upon exposure to an interrogation signal which alternates at the predetermined frequency and is generated by detecting apparatus, and the resonance of the marker is detected by the detecting apparatus. Typically, magnetomechanical markers are deactivated by exposing the bias element to an alternating magnetic field of sufficient magnitude to degauss the bias element. After the bias element is degaussed, the marker's resonant frequency is substantially shifted from the predetermined frequency, and the marker's response to the interrogation signal is at too low an amplitude for detection by the detecting apparatus.
In addition to conventional deactivators utilizing coils excited with an alternating signal, the assignee of the present application has developed additional deactivation devices having advantageous operating characteristics. One of these devices is disclosed in co-pending patent application Ser. No. 08/794,012, filed Feb. 3, 1997 and entitled, "Multi-Phase Mode Multiple Coil Distance Deactivator for Magnetomechanical EAS Marker". The '012 application has common inventors with the present application and discloses a number of embodiments of devices for deactivating magnetomechanical EAS markers. A main point of the disclosure of the '012 application is that the deactivators disclosed therein provide substantial alternating magnetic fields oriented in three mutually orthogonal directions to provide reliable deactivation of markers regardless of the orientation of the markers when presented for deactivation. The deactivation devices of the '012 application also provide for reliable deactivation of markers even when the markers are presented for deactivation at some distance (a matter of inches) from the deactivation device. In one embodiment disclosed in the '012 application, four planar rectangular coils are arranged in a two-by-two array in proximity to each other in a common plane, and the deactivation device is repeatedly switched between two modes of operation. In the first mode of operation, the two coils along one diagonal of the two-by-two array are simultaneously driven in phase opposition to each other, while the other two coils are not driven. In the second mode, the latter two coils are driven in phase opposition to each other, and the first two coils are not driven.
Another co-pending patent application is Ser. No. 08/801,489, filed Feb. 18, 1997, and entitled, "Apparatus for Deactivating Magnetomechanical EAS Markers Affixed to Magnetic Recording Medium Products" and having the same inventors as the present application. In the '489 application there are disclosed deactivation devices which employ planar arrays of "pancake" coils in which field gradients are minimized and generally uniform field levels are provided in proximity to the deactivation device, so that magnetomechanical EAS markers may be reliably deactivated without adversely affecting magnetic medium products to which the markers are affixed.
Although the above-referenced '012 and '489 applications are believed to represent advances over conventional deactivators, the inventors have recognized additional opportunities for improvements in marker deactivation equipment.